A 43 µm $\times$ 269 µm Light-Adaptive Optoelectronic Autonomous Microsystem for Neural Recording

We present a 43 µm ${\boldsymbol{\times}}$ 269 µm tetherless neural recording microsystem in which the CMOS bulk is forward-biased to utilize silicon junctions as a photovoltaic source. Our microsystem, forward-bulk microscale optoelectronic tetherless electrode (FB-MOTE), can operate with as low as 0.2 µA at 0.317 V and can withstand light intensity up to 1200 µW/mm², and is power-adaptive: the higher available power increases the system bandwidth while maintaining the input-referred integrated noise. To balance adaptability and stability, we have designed our amplifier to take up most of the additional power, hence acting like a regulator, while the other circuit blocks are PTAT-biased to remain relatively stable across available power levels. The amplified neural signals are pulse position modulated (PPM) and optically transmitted through an AlGaAs microscale light emitting diode (µLED) for its information-per-photon efficiency, where the µLED driver is designed to maximize the emission-to-area ratio. Finally, we discuss various light-induced effects observed in measurements and introduce a simulation methodology to account for such effects and its limitations. Our forward-bulk CMOS microsystem provides an approach that can effectively harness and account for the available light in optoelectronic systems design.